1. Technical Field
The present invention relates to an electro-optical device, a method of manufacturing the electro-optical device, and an image forming apparatus using the electro-optical device.
2. Related Art
In recent years, a light-emitting device in which electroluminescence (EL) elements are arranged as light-emitting elements has attracted attention. Unlike liquid crystal elements which vary the amount of transmitted light, the EL element is a current-driven light-emitting element which itself emits light, and has a layer contributing to the emission of light such as a hole injection layer or a light-emitting layer.
In manufacturing such a light-emitting device, examples of a method of forming the layer contributing to the emission of light include a method of forming a layer by applying a material of the layer to a wide area on a main substrate by the use of known methods such as a spin coating method, and selectively removing the layer by irradiating a laser beam to an area (portions other than light-emitting portions) not requiring the layer. According to the known method, a film which serves as the layer contributing to the emission of light can be formed over a plurality of light-emitting elements. Accordingly, the known method is excellent In productivity in comparison with a method of forming a layer by selectively applying the material of the light-emitting elements to their formation positions.
In the known method, a UV laser such as an excimer layer or a UV yttrium aluminum garnet (YAG) laser is used as the laser for selectively removing the layer. A laser using krypton fluoride with a wavelength of two hundred and several tens of nanometers can be used as the excimer laser. On the other hand, an infrared laser (for example, CO2 laser) may be used, but since it destroys a layer under the layer to be removed and leaves much processing residue, an infrared laser is not desirable.
A variety of lines to which terminals from an external device for driving or controlling the EL elements are connected are formed on the main substrate. The formation of lines is performed before forming the layer contributing to the emission of light, and the connection of the lines to the terminals from the external device is performed after forming the layer over a wide area. Accordingly, the portions, to which the terminals from the external device are connected, of the lines should be exposed by selectively removing the layer. However, the removal of the layer cannot be performed sufficiently by the use of the UV laser beam. Therefore, it is not possible to connect the lines on the main substrate to the terminals from the external device with sufficient strength and low resistance.
In addition, since it is necessary to connect electrodes (for example, cathodes) formed on the layer contributing to the emission of light to the lines (for example, cathode terminals) on the main substrate, the layer contributing to the emission of light should be removed from the connection positions, thereby exposing the lines. However, when the layer is not sufficiently removed, the resistance of the connection positions increases, thereby not obtaining a desired emission characteristic.
In order to solve the above-mentioned problems, the use of a wet etching method using a liquid agent for selectively removing the layer has been considered. However, since the layer contributing to the emission of light can be immersed in moisture or solvent, the method cannot be employed without being modified. Other examples of the etching method include an oxygen plasma etching method and an ion etching method using nitrogen or argon, but since the methods are isotropic and it is thus difficult to selectively remove the layer by the use of the methods, the methods can be employed without any modification.
Another method of manufacturing a light-emitting device (organic EL device) is described in JP-A-2002-124376. In the method, lines (anode terminal and cathode terminal) are first formed on a main substrate and then a layer is formed by applying a material of a layer contributing to emission of light over a wide area of the main substrate by the use of a spin coating method. Thereafter, by dropping a solvent for dissolving the material and a liquid containing silver, formation of a through-hole reaching the cathode terminal and filling of the through-hole with silver are performed. Then, by forming a cathode layer to cover the through-hole, the cathode layer is connected to the cathode terminal.
However, it is not described in JP-A-2002-124376 that a film which serves as the layer contributing to the emission of light is selectively removed. In the publication, it is described that a light-emitting layer and a hole injection layer are substantially left on the lines of the main substrate after the cathode layer is connected to the cathode terminal with silver, and a process of connecting the lines on the main substrate to terminals of an external device is not described. Even when selective removal of the unnecessary film is attempted by irradiating a laser beam only to an area not requiring the layer on the lines like in the known method, the removal of the film is not sufficient as described above. Accordingly, the connection strength is not sufficient and the resistance is increased.